The present invention relates to an electron gun for a color picture tube and, more particularly, to an in-line electron gun for a color picture tube, wherein focusing characteristics are improved.
In general, a main lens diameter of an electron gun for a color picture tube greatly influences the focusing characteristics. In order to obtain best focusing characteristics, the main lens diameter must be maximized, and mechanical strength must be increased to prevent deformation of the electron gun during assembly.
FIG. 1 is a sectional view showing the main part of a conventional bipotential focusing type in-line electron gun. Reference numerals 1A, 1B and 1C denote cathodes for emitting electron beams from their front ends, respectively; 2, a first grid for controlling the electron beams; 3, a second grid for accelerating the electron beams; and 4, a lower third grid for focusing the electron beams. Reference numerals 2A, 2B and 2C, 3A, 3B and 3C and 4A, 4B and 4C denote apertures for transmitting the corresponding beams therethrough. Reference numeral 5 denotes an upper third grid; and 6, a fourth grid, serving as an anode. Three apertures 5A, 5B and 5C of the upper focus grid 5 oppose three apertures 6A, 6B and 6C of the anode 6 to constitute three main lenses for the electron beams. In this case, operating voltages of 0 V, about 700 V, about 7 kV and about 25 kV are applied to the control grid 2, the accelerating grid 3, the lower and upper focus grids 4 and 5, and the anode 6, respectively.
With the above arrangement, signal potentials at the cathodes 1A, 1B and 1C determine intensities of electron beams, respectively. Three intensity-controlled electron beams A, B and C are slightly focused by prefocus lenses formed by the opposing apertures of the accelerating and lower focus grids 3 and 4. Thereafter, the electron beams are focused by the main lenses constituted by the upper focus grid 5 and the anode 6. The electron beams are focused on a phosphor screen (not shown) of a picture tube. At the same time, the side electron beams A and C are deflected inward at an angle .theta. by means of the apertures 6A and 6C of the anode 6 which are eccentric slightly outward with respect to the apertures 5A and 5C of the upper focus grid 5, thereby converging the electron beams A, B and C to one point. The upper focus grid 5 and anode 6 cannot comprise identical components, resulting in an increase in number of components. In addition, an eccentric assembly jig is required. It should be noted that reference numeral 7 denotes a shield cup.
In a conventional electron gun having the above arrangement, a size of a beam spot (i.e., focusing characteristics) on a phosphor screen of a picture tube must be minimized so as not to degrade sharpness of an image. In order to improve the focusing characteristics, the main lens diameter is conventionally increased.
FIG. 2 is a plan view showing the main part of the upper surface of the upper focus grid 5. Referring to FIG. 2, the three apertures 5A, 5B and 5C each having a diameter D are aligned in line at equal pitches S. As described above, in order to improve the focusing characteristics, the diameter D of each of the apertures 5A, 5B and 5C must be increased. However, the apertures 5A, 5B and 5C of the upper focus grid 5 obtained by pressing a nonmagnetic metal plate such as a stainless steel plate having a thickness of about 0.3 mm must have an aperture structure to increase the withstand voltage between the grid 5 and the anode 6. An aperture depth l must be larger than 1/2 the diameter D to prevent degradation of rotation symmetry of the main lens electric field. The diameter D is smaller by 0.8 to 1.0 mm than the pitch S due to tooling restriction. When the pitch S is increased, convergence errors are increased on the respective dots of the phosphor screen while the picture tube is being operated. A dimension L of the upper focus grid 5 or the anode 6 which constitute the main lenses along the horizontal direction is increased. As a result, the electron gun housed in the bulb is located excessively near the inner surface of the neck, thereby degrading the withstand voltage characteristics.
It is assumed that any error in roundness (major axis-minor axis) of each aperture preferably falls within about 0.5% of the diameter D. For this reason, the electron gun is assembled so that each grid is held on a jig with three core pins (not shown) respectively fitted in the apertures and that heated multiform glass 8 is compressed on a support 9. In this case, each core pin has a diameter smaller by 0.02 to 0.03 mm than the diameter D since errors are present in the pitch S and the diameter D. The manufacturing errors of the respective grids and the deformation of the cup-like body caused by stress upon compression of the multiform glass 8 influence the apertures 5A, 5B and 5C. In a worst case, error in roundness measured upon removal of the grid from the jig is about 0.05 mm. When the diameter D is given as 3.9 mm, the error in roundness is about 1.3%. In this manner, when the error in roundness exceeds tolerance, the electric field of the main lens is distorted, and astigmatism occurs, thereby degrading the focusing characteristics.
FIG. 3 is a sectional view showing the main part of another in-line electron gun having a multi-stage main lens structure. In order to obtain best focusing characteristics in this electron gun, the diameter of an output (i.e., a final stage) main lens must be increased since the electron beam diameter is the largest. Referring to FIG. 3, reference numerals 21A, 21B and 21C denote cathodes for emitting electron beams A, B and C from their front ends; 22, a first grid for controlling the electron beams A, B and C; 23, a second grid for accelerating the electron beams A, B and C; 24, a third grid assembly which comprises a lower third grid 25 and an upper third grid 26; 27, a fourth grid; 28, a fifth grid assembly which comprises a lower fifth grid 29 and an upper fifth grid 30; and 31, a sixth grid. Each grid has three apertures. With this construction, operating voltages of 0 V, about 700 V, about 7 kV and about 25 kV are applied to the first grid 22, the second grid 23, the third and fifth grid assemblies 24 and 28, and the fourth and sixth grids 27 and 31, respectively, in the same manner as in the conventional electron gun described above.
The electron beams A, B and C are slightly focused by prefocus lenses constituted by apertures 23A, 23B and 23C of the second grid 23 and apertures 25A, 25B and 25C of the lower third grid 25. Thereafter, the electron beams are focused by a first main lens assembly L.sub.1 constituted by apertures 26A, 26B and 26C of the upper third grid 26, apertures 27A, 27B and 27C of the fourth grid 27 and apertures 29A, 29B and 29C of the lower fifth grid 29. The electron beams focused by the first main lens assembly L.sub.1 are focused again by a second main lens assembly L.sub.2 constituted by apertures 30A, 30B and 30C of the upper fifth grid and opposing apertures 31A, 31B and 31C of the sixth grid 31. These beams are focused on a phosphor screen (not shown) of a picture tube. In the same manner as described with reference to the first conventional electron gun, the three electron beams are converged to one point. Reference numeral 32 denotes a shield cup.
FIG. 4 is a plan view showing the main part of the upper surface of the upper fifth grid 30 as the final stage of the second main lens assembly L.sub.2. Referring to FIG. 4, the apertures 30A, 30B and 30C are aligned in line at equal pitches S. As described above, in order to improve the focusing characteristics, the diameter D of each of the apertures 30A, 30B and 30C of the fifth grid as the final stage of the main lens unit must be increased.
However, when the edge of the elliptical upper fifth grid 30 comes closer to the inner surface of the neck of the bulb having the electron gun therein, the withstand voltage characteristics are degraded. Therefore, a major axis L of the grid 30 cannot be increased. However, when the diameter of each of the apertures 30A, 30B and 30C is increased without increasing the major axis L, a bridge length d1 between the adjacent apertures is decreased although the length d1 must be more than 0.8 to 1.0 mm. In addition, a distance d2 between the side apertures 30A and 30C the grid periphery along the major axis is decreased. As a result, pressing cannot be easily performed. In addition, the decrease in distance d2 degrades the shielding effect against a change in potential at the inner wall surface of the neck upon operation of the picture tube. The side electron beams A and C are deflected over time to cause convergence errors, and the main lens diameter cannot be easily increased.